ATM Layering

1
ATM Layering

• Does not follow 802 layering
• However, does subdivide the data link layer

ATM
OSI
DataLink
ATM Adaptation Layer (AAL)
CS
SAR
ATM (Application-Independent)
Physical
Physical
2
ATM Layering

• ATM Layer
• Manages delivery of 53-octet cells
• Application-independent Same for all Classes of
  Service

ATM
OSI
DataLink
ATM Adaptation Layer (AAL)
CS
SAR
ATM (Application-Independent)
Physical
Physical
3
ATM Layering

• ATM Adaptation Layer
• Provides what is needed for different classes of
  service
• Application-dependent

ATM
OSI
DataLink
ATM Adaptation Layer (AAL)
CS
SAR
ATM (Application-Independent)
Physical
Physical
4
ATM Layering

• ATM Adaptation Layer is Subdivided Further
• CS (Convergence Services)
• SAR (Segment Assembly/Reassembly)

ATM
OSI
DataLink
ATM Adaptation Layer (AAL)
CS
SAR
ATM (Application-Independent)
Physical
Physical
5
ATM Layering

• Convergence Services (CS)
• Provides whatever services are needed for a
  particular service class (A, B, C, or D)

ATM
OSI
DataLink
ATM Adaptation Layer (AAL)
CS
SAR
ATM (Application-Independent)
Physical
Physical
6
ATM Layering

• Segment Assembly/Reassembly (SAR)
• Does fragmentation and reassembly if CS frame is
  longer than the ATM payload field

ATM
OSI
DataLink
ATM Adaptation Layer (AAL)
CS
SAR
ATM (Application-Independent)
Physical
Physical
7
ATM Layering

• Physical Layer
• Uses SONET/SDH optical fiber transmission
  standard (see Module B)

ATM
OSI
DataLink
ATM Adaptation Layer (AAL)
CS
SAR
ATM (Application-Independent)
Physical SONET/SDH
Physical
8
ATM Cell

• Organization
• 5 octets of overhead, 48 of payload (data)

8
7
6
5
4
3
2
1
Virtual Path Identifier
VPI (cont)
Virtual Channel Identifier
VCI (cont)
res
CLP
VCI (cont)
PT
Header Error Control
Payload (48 octets)
9
ATM Cell

• Virtual Circuit Number is Hierarchical
• Virtual Path Identifier Identifies a site
• Virtual Channel Identifier Identifies a station
  at the VPI site

8
7
6
5
4
3
2
1
Virtual Path Identifier
VPI (cont)
Virtual Channel Identifier
VCI (cont)
VCI (cont)
10
ATM Cell

• Payload Type (2 bits) identifies cell category
• 00 user information (data)
• reserved for later definition (1 bit)

8
7
6
5
4
3
2
1
Virtual Path Identifier
VPI (cont)
Virtual Channel Identifier
VCI (cont)
res
CLP
VCI (cont)
PT
Header Error Control
11
ATM Cell

• Cell Loss Priority Bit
• 0 if sending within committed information rate
• 1 if sending faster switches will discarded if
  congestion occurs

8
7
6
5
4
3
2
1
Virtual Path Identifier
VPI (cont)
Virtual Channel Identifier
VCI (cont)
res
CLP
VCI (cont)
PT
Header Error Control
12
ATM Cell

• Header Error Control Field
• To check the header (not the payload) for errors
• Discard cell if an error is found

8
7
6
5
4
3
2
1
Virtual Path Identifier
VPI (cont)
Virtual Channel Identifier
VCI (cont)
res
CLP
VCI (cont)
PT
Header Error Control
13
ATM Overhead

• ATM has very high overhead
• ATM Layer 5 octets of overhead for 48 octets of
  data
• CS and SAR layers have their own frames, whose
  headers take up room in the payload field
• Even worse, in streaming mode, for applications
  that need low latency, cells are sent out without
  waiting for the SAR frame data fields to be
  filled to capacity